Touch key is a kind of touch sensing input device, which is widely used in cell phone, television and other media devices. According to touch sensing principle, touch keys can be divided into resistive touch keys, capacitive touch keys and so on. Capacitive touch keys draw wide attentions in the industry, due to their high light transmittance, extended life, high resistance to abrasions, environment temperature and humidity variations, and their improved performances such as multi-point touch function. The capacitive touch key can detect a touch event from a finger through insulating materials, e.g., glass or plastic, and determine whether the touch event is an effective one without mechanical buttons as in conventional keys. This process is based on following theories: an increment in capacitance, generated by a finger touching a touch key's sensing area, is added to an inherent capacitance amount of the touch key device to obtain a new capacitance amount, and a touch detection control circuit may process the new capacitance amount to generate corresponding control commands, thereby realizing control.
According to power supply, touch keys can be divided into primary and secondary coil common ground touch keys and primary and secondary coil non common ground touch keys. Referring to FIG. 1, for a primary and secondary coil common ground touch key, when the key is being touched by a finger, a body, as a large capacitor CF, may be connected to the ground. Meanwhile, a sensing capacitor CP of the key may connect with the ground as well. As a result, the capacitor CF and CP are in parallel. Referring to FIG. 2, for a primary and secondary coil non common ground touch key, when the key is being touched by a finger, a body, as a large capacitor CF, may be connected to the ground. And a sensing capacitor CP of the key may connect with a direct current ground. Thus, the capacitor CF and CP are not in parallel.
In conventional methods, key detection is always performed based on capacitance variation to ground. In no ground situation, current leak may occur. If touch detection is still performed by employing a conventional method, a false detection result may be obtained, leading to false key detection.
Therefore, a new method for detecting key more accurately is desired.